A preferred application for the present invention is in high voltage three phase circuit breakers. Therefore, the background of the invention is described below in connection with such devices. However, it should be noted that, except where they are expressly so limited, the claims at the end of this specification are not intended to be limited to applications of the invention in a high voltage three phase circuit breaker. For example, the invention disclosed herein may be employed in association with a circuit switcher, circuit breaker, load break switch, recloser, or the like.
A high voltage circuit breaker is a device used in the transmission and distribution of three phase electrical energy. When a sensor or protective relay detects a fault or other system disturbance on the protected circuit, the circuit breaker operates to physically separate current-carrying contacts in each of the three phases by opening the circuit to prevent the continued flow of current. In addition to its primary function of fault current interruption, a circuit breaker is capable of load current switching. A circuit switcher and a load break switch are other types of switching device. As used herein, the expression "switching device" encompasses circuit breakers, circuit switches, load break switches, reclosers, and any other type of electrical switch.
The major components of a circuit breaker or recloser include the interrupters, which function to open and close one or more sets of current carrying contacts housed therein; the operating mechanism, which provides the energy necessary to open or close the contacts; the arcing control mechanism and interrupting media, which interrupt current and create an open condition in the protected circuit; one or more tanks for housing the interrupters; and the bushings, which carry the high voltage electrical energy from the protected circuit into and out of the tank(s) (in a dead tank breaker). In addition, a mechanical linkage connects the interrupters and the operating mechanism.
Circuit breakers can differ in the overall configuration of these components. However, the operation of most circuit breakers is substantially the same. For example, a circuit breaker may include a single tank assembly which houses all of the interrupters. U.S. Pat. No. 4,442,329, Apr. 10, 1984, "Dead Tank Housing for High Voltage Circuit Breaker Employing Puffer Interrupters," discloses an example of the single tank configuration. Alternatively, a separate tank for each interrupter may be provided in a multiple tank configuration. An example of a multiple tank circuit breaker is depicted in FIG. 1.
As shown in FIG. 1, the circuit breaker assembly 1 includes three cylindrical tanks 3. The three cylindrical tanks 3 form a common tank assembly 4 which is preferably filled with an inert, electrically insulating gas such as SF.sub.6 . The tank assembly 4 is referred to as a "dead tank" because it is at ground potential. Each tank 3 houses an interrupter (not shown). The interrupters are provided with terminals which are connected to respective spaced bushing insulators. The bushing insulators are shown as bushing insulators 5a and 6a for the first phase; 5b and 6b for the second phase; and 5c and 6c for the third phase. Associated with each pole or phase is a current transformer 7. In high voltage circuit breakers, the pairs of bushings for each phase are often mounted so that their ends have a greater spacing than their bases to avoid breakdown between the exposed conductive ends of the bushings. Such spacing may not be required in lower voltage applications. The operating mechanism that provides the necessary operating forces for opening and closing the interrupter contacts is contained within an operating mechanism housing 9. The operating mechanism is mechanically coupled to each of the interrupters via a linkage 8.
A cross section of an interrupter 10 is shown in FIGS. 2A-C. The interrupter provides two sets of contacts, the arcing contacts 12 and 14 and the main contacts 15 and 19. Arcing contacts 12 and main contacts 19 are movable to close or open the circuit. FIG. 2A shows a cross sectional view of the interrupter with its contacts closed whereas FIG. 2C shows a cross section of the interrupter with the contacts open.
The arcing contacts 12 and 19 of high voltage circuit breaker interrupters are subject to arcing or corona discharge when they are opened or closed. As shown in FIG. 2B, an arc 16 is formed between arcing contacts 12 and 14 as they are moved apart. Such arcing can cause the contacts to erode and disintegrate over time. Current interruption must occur at a zero current point of the current waveshape. This requires the interrupter medium to change from a good conducting medium to a good insulator or non-conducting medium to prevent current flow from continuing. Therefore, a known practice (used in a "puffer" interrupter) is to fill a cavity of the interrupter with an inert, electrically insulating gas that quenches the arc 16. As shown in FIG. 2B, the gas is compressed by a piston 17 and a jet or nozzle 18 is positioned so that, at the proper moment, a blast of compressed gas is directed toward the arc, extinguishing it. Once formed, an arc is extremely difficult to extinguish it until the arc current is substantially reduced. Once the arc is extinguished as shown in FIG. 2C, the protected circuit is opened, preventing current flow.
Circuit breakers can switch various devices in the electric utility system. Primarily, these devices include transmission lines, transformers, shunt capacitor banks, and shunt reactors. All circuit breaker switching operations generate closing or opening tranients in the system as the system adjusts to the new set of operating conditions as a result of the switching operation. Synchronization of circuit breaker closing and opening to system voltage and current waveforms can drastically reduce these transients and, in addition, reduce interrupter wear. For example, shunt capacitor banks are used in utility systems to regulate system voltages as load levels and system configuration changes occur.
Voltage and current transients generated during the energization of shunt capacitor banks have become an increasing concern for the electric utility industry. The concern relates to power quality for voltage-sensitive loads and excessive stresses on power system equipment. For example, modern digital equipment requires a stable source of power. Moreover, computers, microwave ovens, and other electronic appliances are prone to failures resulting from such transients. Even minor transients can cause the power waveform to skew, rendering these electrical devices inoperative. Therefore, utilities have set objectives to reduce the occurrence of transients and to provide a stable power waveform.
Conventional solutions for reducing the transients resulting from shunt capacitor energization include circuit breaker pre-insertion devices, for example, resistors or inductors, and fixed devices, such as current limiting reactors. While these solutions provide varying degrees of success in reducing capacitor bank energization transients, they result in added equipment, added cost, and added reliability concerns.
The maximum shunt capacitor bank energization transients are associated with closing the circuit breaker at the peak of the system voltage waveform, where the greatest difference exists between the bus voltage, which will be at its maximum, and the capacitor bank voltage, which will be at a zero level. Where the closings are not synchronized with respect to the system voltage, the probability for obtaining the maximum energization transients is high. One solution to this problem is to synchronously close the circuit breaker at the instant the system voltage is substantially zero. In this way, the voltages on both sides of the circuit breaker at the instant of closure would be nearly equal, allowing for an effectively "transient-free" energization.
While the concept of synchronous or controlled switching is a simple one, a cost-effective solution has been difficult to achieve, primarily due to the high cost of providing the required timing accuracy in a mechanical system. One solution is to use three separate operating mechanisms and corresponding linkages to synchronously control the operation of each pole individually. U.S. Pat. No. 4,417,111, Nov. 22, 1983, entitled "Three-Phase Combined Type Circuit Breaker," discloses a circuit breaker having a separate operating mechanism and associated linkage for each of the three phases or poles. However the use of three separate operating mechanisms and associated linkages is expensive and increases the overall size and complexity of the circuit breaker.
U.S. Pat. No. 4,814,50, Mar. 21, 1989, "High Voltage Circuit Breaker" (assigned to Asea Brown Boveri AB, Vasteras, Sweden) discloses a device for synchronously closing and opening a three phase high voltage circuit breaker so that a time shift between the instants of contact in the different phases can be brought about mechanically by a suitable choice of arms and links in the mechanical linkage. This linkage uses an a priori knowledge of the time required to close and open the interrupter contacts in each of the three phases. The time differences can be accounted for by an appropriate design of the mechanical linkage. However, such a linkage cannot support dynamic or adaptive monitoring of the voltage waveform of each phase to achieve independent synchronization. Moreover, the mechanical linkage disclosed would require mechanical adjustments over time to account for variations in the circuit breaker performance and operating conditions which often change over time.